Sylvie, As far as 18-200VR goes, I don't think that it will be a good candidate for underwater use. For one thing, minimum focusing distance is 50 cm so it won't be all that useful for macro unless you use strong diopter.
Secondly, port will be very tricky. When fully extended at 200mm end, it is about twice the length of the lens at 18mm lens. If you use flat port, you can pretty much use only the telephoto end as you will see part of the ports at wide angle end. If you use a domeport, when properly position for wide angle end , there won't be enough clearance in the dome for the lens to fully extend. So I think this will be pretty much a land lens only.
As far as my wish list, I think it is not hard to figure from my sig, certain items that are missing
Nuff said before I get into trouble and they won't tell me anymore inside scoop